This disclosure relates to an apparatus and method for evaluating fluids encountered in a well services context using x-rays. More specifically, this disclosure relates to a system for using x-rays to determine the density and phase fractions of a well services fluid such as a fracturing fluid, a cement slurry, a mixture of drilling mud and cuttings, or any other fluid that may be encountered. These measurements are generally taken above ground using an x-ray generator and a measurement radiation detector with the fluid of interest being housed in a pipe. Additionally, a second reference radiation detector may be used that detects a filtered signal from the x-ray generator and controls an accelerating voltage and a beam current of the x-ray generator.
It is common in the recovery of hydrocarbons from subterranean formations to fracture the hydrocarbon-bearing formation to provide flow channels through which the desired fluid can be obtained. In such operations, a fracturing fluid is injected into a wellbore penetrating the subterranean formation and is forced against the formation strata by pressure. The formation strata or rock is forced to crack or fracture, and a proppant is placed in the fracture by movement of a viscous fluid containing proppant into the crack of the rock. The resulting fracture, with proppant in place, provides improved flow of the recoverable fluid, i.e., oil, gas, or water, into the wellbore.
Fracturing fluids often comprise a thickened or gelled aqueous solution which has suspended therein proppant particles that are substantially insoluble in the fluids of the formation. Proppant particles carried by the fracturing fluid remain in the fracture created, thus propping open the fracture when the fracturing pressure is released and the well is put into production. Suitable proppant materials include sands (silicon, ceramic, resin), walnut shells, sintered bauxite, glass beads, salts, or similar materials. The propped fracture provides a larger flow channel to the wellbore through which an increased quantity of hydrocarbons can flow.
In the industry, it is desirable to monitor the quality of the fluid within the system. This includes monitoring the concentration of particulates within the fluid. Current methods for controlling the quality of the addition of particulates include: pre and post-job batch weighing, mechanical metering during the addition of particulates, or radioactive measurements of the fluid slurries during operations.
Batch weighing provides quality control of the cumulative total product used, but does not provide quality control during on the fly operations for pre-engineered programs that vary the rate at which particulates are added during different phases of the injection.
Mechanical metering involves measuring the rate at which the particulate is added and the rate of the fluid prior to addition (clean rate) and then using these rates to calculate the particulate concentration of the slurry. The calculation for concentration is based on the knowledge of the density of the fluid and the particulate material. However, mechanical metering is prone to slippage and inaccuracies due to the efficiencies of the mechanical system being employed. The quality of the measurement is therefore limited.
The density of fracturing fluids has been determined using radioactive systems as well. Specifically, gamma-ray densitometers are currently used in the oilfield for controlling the proppant mass balance in fracturing jobs. The basic measurement is the attenuation of Cesium (Cs137) 662 keV gamma-rays by the fracturing fluid. With proper calibration and data processing, the proppant mass balance error is in the range of 1-2%. This type of system takes a single measurement of the radiation flux reaching the detector and determines a density from this measurement.
While this type of system can provide an accurate result, there are drawbacks to the use of a chemical source such as Cs137 in measurements in the field. Any radioactive source carries high liability and strict operating requirements. These operational issues with chemical sources have led to a desire to utilize a safer radiation source. Although the chemical sources do introduce some difficulties, they also have some significant advantages. Specifically, the degradation of their output radiation over time is stable allowing them to provide a highly predictable radiation signal. An electrical radiation generator would alleviate some of these concerns, but most electrical photon generators (such as x-ray generators) are subject to issues such as voltage and beam current fluctuation. If these fluctuations can be controlled, this would provide a highly desirable radiation source.
In addition to measuring the density of fracturing fluid, it is also useful to measure properties of other fluids utilized in the oilfield. For instance, when production on a well comes to a close, it is necessary to fill the well with a cement slurry to stabilize the remaining fractures surrounding the well. It is desirable to use the same tool used for fracturing fluid density determination to determine the phase fractions of water and cement in the cement slurry. Prior art systems for phase fraction determination have also utilized chemical sources which may not be desirable for the reasons detailed above.
Accordingly, a need has been identified for a tool that may be used to determine properties of any fluid encountered in the well services context. One specific example is to measure the density of fracturing fluid employing an electrical photon generator such as an x-ray generator. This generator must be stable over time with its parameters closely controlled to ensure accurate measurements regardless of changing conditions. Additionally, it is desired to use the same system to determine the phase fractions of cement and water in a cement slurry or the characteristics of any other well services fluid that may be encountered.